1. Field of the Invention
This invention relates to a method for reproducing an image on a recording material, such as a photosensitive material, and an apparatus for carrying out the method. This invention particularly relates to an image reproducing method, wherein a recording light beam is caused to scan a recording material by a light beam scanning system, which includes a rotating polygon mirror, and an apparatus for carrying out the method.
2. Description of the Prior Art
Image reproducing methods, wherein light beam scanning systems are utilized to scan recording light beams, have heretofore been known. In such an image reproducing method, a light beam is caused by a light beam scanning system to two-dimensionally scan a recording medium, on which an image has been recorded. Light, which carries information about the image, is thereby radiated out of part of the recording medium, which is being exposed to the light beam. The light, which has been radiated out of the recording medium, (i.e., light reflected by the recording medium, light which has passed through the recording medium, or light emitted by the recording medium) is detected by a photoelectric read-out means, and an image signal is thereby obtained which is made up of a series of image signal components representing the image recorded on the recording medium. Also, a light beam is modulated in accordance with the image signal, which has thus been obtained, and is caused by the light beam scanning system to two-dimensionally scan a recording material. In this manner, the image is reproduced on the recording material. Such a method is disclosed in, for example, U.S. Pat. No 4,975,580.
As described above, in the aforesaid image reproducing methods, light beam scanning systems are utilized to scan recording light beams. A light beam scanning system is ordinarily constituted of a mechanical light deflector, which causes the light beam to scan in a main scanning direction, and a sub-scanning means for conveying a recording media, on which an image has been recorded, or a recording material, on which an image is to be reproduced, in a sub-scanning direction, which is approximately normal to the main scanning direction. In many cases, rotating polygon mirrors which have a plurality of reflecting surfaces are used as the mechanical light deflectors. A rotating polygon mirror is advantageous over a galvanometer mirror in that the speed at which the light beam is deflected can be increased, which allows an image reproducing operation to be carried out quickly.
However, the rotating polygon mirror has problems in that the reflectivity fluctuates among the reflecting surfaces and, therefore, nonuniformity in the intensity of the scanning light beam occurs. The rotating polygon mirror also has problems in that the inclination with respect to the rotation axis of the rotating polygon mirror and the distance therefrom differ among the reflecting surfaces. Additionally, the speed at which the rotating polygon mirror is rotated fluctuates periodically, so that the speed at which the light beam is deflected differs among the reflecting surfaces of the rotating polygon mirror. If the intensity of the light beam and the speed, at which the light beam scans a recording material, fluctuate, nonuniformity in the image density will occur in the image reproduced on the recording material, and the image quality of the reproduced image cannot be kept good.
Accordingly, a method has been proposed to prevent the nonuniformity in the image density from occurring in the reproduced image. In the proposed method, when an image is to be reproduced on a recording material, a light beam modulated in accordance with image signal components of an image signal, which have been obtained from each scanning line during the operation for reading out an image recorded on a recording medium, is utilized to scan along a plurality of scanning lines on the recording material. (Such a scanning operation will hereinbelow be referred to as the multiple type of scanning operation.) In this manner, the scanning density with respect to the sub-scanning direction, which is normal to the scanning lines, is set at a higher density and the nonuniformity in the image density in the image reproduced on the recording material is rendered less perceptible than when the light beam modulated in accordance with the image signal components, which have been obtained from each scanning line on the recording medium, is utilized to scan along a single scanning line on the recording material.
FIG. 3 is a graph showing perceptibility characteristics of a difference in image density. Specifically, levels of the difference .DELTA.D in the image density capable of being perceived by a person from a pattern, in which the image density changes in a sinusoidal form within the range of the difference .DELTA.D in the image density, were determined with respect to the spatial frequency. FIG. 3 shows an example of the results of the determination. In FIG. 3, the region above curve "a" is the one in which the difference .DELTA.D in the image density can be perceived. The region below curve "b" is the one in which the difference .DELTA.D in the image density cannot be perceived. The region between curve "a" and curve "b" is the one in which the difference .DELTA.D in the image density can be or cannot be perceived, depending on the person who observes the image density pattern and the apparatus used for the determination. As illustrated in FIG. 3, in the region of the spatial frequency higher than approximately 0.5 line/mm (in the cases of ordinary image reproducing operations, the pitch of the scanning line falls within this region), the minimum value of the difference .DELTA.D in the image density, which can be perceived, becomes larger as the spatial frequency is higher. Specifically, as the spatial frequency is higher, only a higher level of the difference .DELTA.D in the image density can be perceived. Therefore, when the pitch of the scanning line is set at a small value (i.e., when the scanning density is increased) by carrying out the multiple type of scanning operation, the nonuniformity in the image density of a reproduced image becomes less perceptible.
Methods for rendering the nonuniformity in the image density of a reproduced image imperceptible by carrying out the multiple type of scanning operation have been proposed in, for example, U.S. Pat. Nos. 4,363,037 and 4,775,896 and Japanese Unexamined Patent Publication No. 3(1991)-49471.
However, when the nonuniformity in the image density of a reproduced image is rendered imperceptible by carrying out the multiple type of scanning operation, a single scanning line on the recording medium, on which the image has been recorded, is replaced by a plurality of scanning lines on the recording material, on which the image is to be reproduced. Therefore, problems occur in that each scanning line composed of the plurality of the scanning lines does not have a sharp appearance on the recording material. Specifically, as the number of the plurality of the scanning lines on the recording material, which replace a single scanning line on the recording medium, becomes larger, the nonuniformity in the image density of the reproduced image becomes less perceptible, but the sharpness of the reproduced image becomes lower.